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Green water and blanketweed are the two most common pond problems in the UK, and almost every pond owner battles them at some point, usually from April to September. Both are forms of algae, both are fed by the same things, and both can be controlled. The difference is in how you tackle them.
Green water vs blanketweed
| Green water | Blanketweed | |
|---|---|---|
| What it is | Billions of single-celled algae suspended in the water | Long strands of filamentous algae |
| What it looks like | Pea-soup water; you can't see more than a few centimetres down | Hair-like or cotton-wool mats on rocks, waterfalls and plants |
| Best long-term fix | UV clarifier plus good filtration | Lower nutrients, plants and a blanketweed treatment |
Interestingly, clearing green water often triggers blanketweed. Once the water is clear, sunlight reaches the bottom and the strand-forming algae take advantage of the nutrients the green water was using.
What causes pond algae
Algae need three things: light, nutrients and warm water. You can't control the weather, but you can reduce the light and nutrients.
- Excess nutrients: nitrate and phosphate from fish waste, uneaten food, rotting leaves and sludge, fertiliser running in from lawns and borders, and tap water top-ups.
- Too much sun: a pond in full sun with little plant cover is far more prone to algae.
- Undersized filtration: a filter that can't keep up with the fish load leaves waste in the water.
- Too many fish, or too much food: especially in ponds with koi.
- New ponds: almost every new pond goes green in its first season while the filter and plants establish.
How to clear green water
1. Fit or check a UV clarifier
A UV clarifier is the single most effective answer to green water. Water is pumped past an ultraviolet lamp, which causes the floating algae cells to clump together so the filter can trap them. Most ponds clear within one to two weeks of a correctly sized unit being fitted.
- Size the UV for your pond volume and fish stock. Units such as the TMC Pro Clear Ultima 30w or an all-in-one such as the Oase Filtral UVC 6000 suit many garden ponds.
- Replace the bulb every year, ideally each spring. UV bulbs keep glowing long after they've stopped being effective. We stock replacement lamps such as the Evolution Aqua T8 UV Bulb 30w and TMC UV-C Compact Lamp 55w.
- Clean the quartz sleeve when you change the bulb. A coated sleeve blocks the UV light.
Browse pond UV clarifiers and bulbs.
2. Make sure your filter is big enough
The UV clumps algae together, but the filter has to remove it. Your filter should be rated for at least your pond's volume, and more if you keep koi. Clean the mechanical sponges regularly in pond water. See our pond filters and read our guide to pond filters.
3. Use a clarifier for a quick boost
A flocculant such as Eco Pond Cloudy Water Treatment clumps fine particles together so your filter can remove them faster. It helps while the UV is getting to work. See our cloudy pond water treatments.
How to get rid of blanketweed
- Remove it by hand. Twist it around a stick or garden cane like candyfloss, and lift it out. Leave it on the side of the pond for a day so small creatures can crawl back in.
- Treat the pond. Blanketweed treatments starve the algae of nutrients or change the conditions it needs. Evolution Aqua Stop Blanketweed and Cloverleaf Blanket Answer are both popular choices. For an ongoing preventative, Cloverleaf Prevent Blanketweed is designed to be used through the season.
- Remove dying algae. As treated blanketweed dies back, net it out. Leaving large amounts to rot will feed the next bloom and use up oxygen.
- Add aeration while treating. Decomposing algae uses oxygen, especially on warm nights. An air pump helps protect your fish.
Liquid algae treatments such as Tetra Pond AlgoFin work on both green water and blanketweed. Always dose by the pond's actual volume and follow the label carefully, particularly in hot weather when oxygen levels are lower.
Long-term prevention
Reduce nutrients
- Feed fish only what they eat in five minutes.
- Net leaves out, and vacuum the bottom once or twice a year.
- Use a natural sludge digester such as Eco Pond Mud Muncher to break down organic waste on the pond floor.
- Top up with rainwater where you can; tap water often contains phosphate and nitrate.
- Keep fertilisers and lawn run-off away from the pond.
Compete with algae using plants
Aim for around 50–60% of the surface covered by lily pads and floating plants during summer. Plants shade the water and use the same nutrients the algae need. Oxygenating plants are especially good at this. See our pond planting range and our pond planting guide. A floating planter such as the Velda Floating Plant Oasis adds shade and filtering roots in deep water.
Use barley-based treatments
Barley extract treatments such as Eco Pond Barley-Bio Algae Control are a gentle, natural way to discourage algae through the season. They work best as a preventative from early spring rather than a cure for a bloom.
A quick troubleshooting checklist
- Still green after two weeks of UV? Check the bulb's age, clean the quartz sleeve and check the flow rate isn't too fast for the unit.
- Clear water, then blanketweed? That's normal. Focus on nutrients and plants, and treat if needed.
- Brown, cloudy water rather than green? That's usually suspended dirt or sludge rather than algae. A clarifier, filter maintenance and a pond vacuum are the fix.
Browse all pond sludge and algae treatments, or read The Complete Guide to Pond Treatments for a full-year plan.
